1. Field of the Invention
The present invention relates to a degreasing solution and a degreasing method for degreasing surfaces of metal materials such as iron, zinc and aluminum, and non-metal materials such as plastic.
2. Description of the Background Art
In the field of metal surface treatment or the like, a degreasing treatment is generally performed on an object for removing fat and oil such as mineral oil and animal and vegetable oils adhering to its surface, as a pretreatment for chemical conversion through phosphating or the like. A degreasing agent employed for such a degreasing treatment generally contains a builder which is mainly composed of acid or alkali and a nonionic or anionic surfactant as its main component. While an alkali builder which is mainly composed of phosphate or silicate has been generally employed in consideration of detergency, an alkali builder which is mainly composed of silicate is recently employed by preference, since phosphate causes enrichment of lakes and marshes or the like. An alkali builder which is prepared by further blending silicate with carbonate or the like is also employed.
While a nonionic or anionic surfactant is employed as described above, a nonionic surfactant is generally employed since an anionic surfactant has high foamability.
An HLB value is generally known as a value indicating the characteristics of the nonionic surfactant. This HLB value indicates the balance between the hydrophilic and hydrophobic groups of the surfactant, and the number of hydrophilic group which is contained in the surfactant increases as the HBL value increases. In case of alkaline-degreasing a metal surface or the like, a nonionic surfactant having a high HLB value is generally employed. This is because the detergency increases with the HLB value, and a nonionic surfactant having an HLB value of at least 12 is generally employed in a conventional alkaline degreasing agent. However, a nonionic surfactant having a high HLB value, which has a strong foaming tendency in general, is generally employed together with a defoaming agent having a low HLB value.
In such a conventional degreasing agent, however, oil such as mineral oil, machine oil or spindle oil which is mixed into the degreasing solution by alkaline degreasing form an extremely stable emulsion, due to the action of the surfactant. Consequently, the degreasability of the surfactant disadvantageously extremely reduces when the concentration of the oil which is mixed into the degreasing agent increases.
Particularly when alkaline degreasing is performed under a condition of sufficiently stirring and mixing the degreasing agent, a stable emulsion is readily formed by the surfactant and the mixed oil, to cause such a problem that the mixed oil is not separated but the oil contamination resistance reduces. An example of such a stirring method is a supervibration stirring method which is disclosed in Japanese Patent Publication No. 6-71544 (1994). This supervibration stirring method is adapted to set diaphragms in a degreasing treatment vessel and vibrate the same under conditions of prescribed vibration width, frequency etc. thereby vibrating and stirring a degreasing solution which is stored in the degreasing treatment vessel, for example. When a degreasing treatment is performed by such a supervibration stirring method with a conventional alkaline degreasing solution, however, an extremely fine and stable emulsion is formed and oil cannot be readily separated from water after alkaline degreasing.